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436 ALASKAN GLACIER STUDIES
portion of the glacier cover the whole surface thickly, except in areas of slumping. They include alders at least 18 years of age.
The terminal hill rises f rqm the water's edge hi Copper River in a steep slope, west of which there is a long gradual ascent to an elevation of about 400 feet and a steep descent over a westward-facing cliff to the interior flat.
Much of the morainic material covering the ice of the terminal hill is angular and the bowlders are exceptionally large. They include many rock masses 12 to 20 feet on a side. Some of these huge rock masses in the railway cut on the eastern edge of the glacier are so large that they lead to the impression that they are rock ledges in place. This is not the case, however, as was ascertained by careful study in 1910. None of the other glaciers of the lower Copper River are encumbered with such large de'bris, except the western terminus of Miles Glacier at Abercrombie Rapids. This may possibly be explained by the exceedingly steep walls of the valley of Grinnell Glacier,
FIQ. 67. NATURAL SOAIJB LONGITUDINAL SUCTION or GBINNELL GLACHIB IN 1910.
upon which avalanches should be prevalent. As such coarse material is not found in the present lateral moraines of the cascading portion of Grinnell Glacier, however, it may be that the coarse angular debris was avalanched down upon the Grinnell Glacier during some great earthquake in the past and carried forward to the terminal bulb in the subsequent advance of the glacier.
The Interior Fled. Between the terminal hill and the cascading portion of Grinnell Glacier is an interior flat (PL CLXIX) between a quarter and a half mile square. Its surface is covered with ablation moraine and outwash gravel, overlying the ice, which is nowhere exposed. As Fig. 67 shows, the expanded part of Grinnell Glacier may possibly constitute a detached ice block, although it seems more likely that the ice is continuous beneath the ablation moraine and outwash, for areas of slumping are found here and there between the terminal hill and the clean ice of the cascading portion of the glacier.
The interior flat consists 6f three different portions. In the middle is a broad, low ridge of irregular ablation moraine. On each side of this are alluvial fans of outwash gravels.
The medial ridge rises 50 to 100 feet above the flanking alluvial fans. Its surface is made up of angular ablation moraine; the bowlders being decidedly smaller than in the terminal hill. The water which oozes out here and there shows that ice underlies the moraine at no great depth. Compared with the terminal hill this portion of the interior flat is comparatively barren, but small scattered shrubs are growing upon different portions of the surface. None of them are as old as those on the terminal hill.
North of this medial ridge is a detached ice block, which lies upon the northern valley wall. It is separated from the medial ridge by outwash gravels. Its surface is completely mantled by ablation moraine upon which a few small shrubs are growing.